09-09-2013, 04:13 PM
DISTRIBUTED FILE SYSTEM-ENHANCEMENT AND FURTHER DEVELOPMENT
DISTRIBUTED FILE SYSTEM.pptx (Size: 71.11 KB / Downloads: 37)
Distributed File System
A distributed file system is a client/server based application that allows clients to access and process data stored on the server.
Sun Microsystems' Network File System (NFS), Novell NetWare, Microsoft's Distributed File System, and IBM/Transarc's DFS are some examples of distributed file systems.
Enhancement
Several advances have been made in the design of distributed file systems since the emergence of NFS and AFS.
Advances that enhance the performance, availability and scalability of conventional distributed file systems.
NFS Enhancements
Several research projects have addressed the need for one-copy update semantics by extending the NFS protocol to include open and close operations and adding a callback mechanism to enable the server to notify clients of the need to invalidate cache entries.
NFS developer have been directed at making NFS servers more accessible and useful in wide-area networks.
Achieving one-copy update semantics.
The stateless server architecture of NFS brought great advantages in terms of robustness and ease of implementation.
It prevents the achievement of precise one-copy update semantics.
It also prevents the use of callbacks.
Spritely NFS
Clients’ modules must send an open operation on the server.
The parameters of the Sprite open operation specify a mode(read, write or both) and include counts of the number of local processes.
when a local process closes a remote file, a close operation is sent to the server with updated counts of readers and writers.
NQNFS
Aim to add more precise cache consistency to the NFS protocol and to improve performance through better use of caching.
It maintains similar client-related state concerning open files, but it uses leases to aid recovery after a server crash.
The server sets an upper bound on the time If the client wishes to continue beyond that time, it must renew the lease.
if the clients don’t
reply, the server simply waits until their leases expire before responding to the new write request.
AFS enhancements
In AFS, callbacks are generated only when the server receives a close operation for a file that has been updated.
In order to update a file, a client must obtain a write token from the server, specifying a range of bytes in the file that the client is permitted to update.
When a write token is requested, clients holding copies of the same file for reading receive revocation callbacks.
All tokens have an associated lifetime, and clients must renew them after their lifetime has expired.